Chemistry Reference
In-Depth Information
Cao et al. used P-,B- and N-doped CNT for aerobic oxidation of cyclo-
hexane.
114
N- and P- doping improved the activity of CNTs in cyclohexane
oxidation due to the enhanced electron transfer between graphene sheets
and reactive radicals from n-type dopants such as N and P. B-doping does
not have the same electron transfer effect due to its electron-deficiency
and will not improve the activity of CNT.
2.4 Boron-functionalised CNT/CNF
Doping graphite,
110,113,115,116
carbon/carbon composite,
117
or other car-
bon materials
118,119
with boron has also been extensively studied in order
to alter the electronic properties
71,119
and increase the resistance of
carbon.
23,110,119
Boron substitution has also been applied to improve
carbon's capacity as anode material in Li ion batteries.
120-122
Recently,
boron also has been introduced into carbon nanotubes
71
to change their
electronic properties and electrochemical behaviour. However, the effect
of boron doping on the oxidation kinetics is more complicated than the P
doping. Information available in the literature concerning the effects of
boron compounds on the oxidation kinetics of carbon is somewhat
contradictory because both catalytic
112
and inhibiting effects
112,115
or
very less inhibiting effects
117
have been described.
Different methods have been employed to dope the carbon materials,
Boric oxide B
2
O
3
and ammonium borate (NH
4
)
2
B
10
O
16
8H
2
O in hot
aqueous are often used for impregnation of carbon. Benzene boronic
acid C
6
H
5
B(OH)
2
and ttimethylamine borane (CH
3
)3NBH
3
) can be ap-
plied in ethanol solution and the liquid alkyl borates trimethyl-, triethyl-,
tri-tert-butyl-, and triamyl borate can also be directly added to the
graphite powder to the point of incipient wetness.
XPS is typically used to identify the B state in the carbon as a function
of treatment conditions. The assessment of XPS peaks are summarized in
the Table 3.
Boron as a dopant was substantiated to enhance the graphitization of
carbon as confirmed by XRD. The XPS results confirmed that the doped
boron is a substituent. It is oxidized during carbon oxidation and
segregated into the surface and then it remains on the carbon surface as
boron oxide.
McKee et al. suggested a structure of B
2
O
3
on carbon surfaces.
113
Glassy B
2
O
3
contains randomly oriented three-dimensional networks
of BO
3
groups, each oxygen atom being bonded to three boron atoms.
The borate group, BO
3
in, for example, tri-methyl borate, has a planar
trigonal configuration with a B-O bond distance of 1.38 Å and a O-B-O
bond angle of 1201. Figure 11 shows a schematic two-dimensional
Table 3 Assessment of B XPS peaks.
XPS peaks
Assessment
Ref.
186.5 ev
Substitutional B
118
193.2 ev
B
2
O
3
123
188.1 ev
B
x
C
1
x
118
